Receptor tyrosine kinases (RTK) are single-pass transmembrane receptors expressed on the plasma membrane. Upon the binding of an extracellular signalling molecule (e.g. growth factors, hormones), RTKs dimerize leading to the activation of the intracellular tyrosine kinase domain and intermolecular phosphorylation. The phosphotyrosines function as specific sites for the assembly, phosphorylation and activation of downstream signaling molecules. (Adapted from PMID:20602996).